Operational amplifiers with long-time constant feedback networks are useful for many purposes. One application of these devices is that of amplifying and integrating the signal from a rate-dependent transducer, such as a search coil or magnetic vibration pickup, in order to obtain an output which is proportional to the driving function rather than to its time derivative. In some such applications, it is necessary that the signal response of the device extend to very low frequencies. Furthermore, in order to minimize phase shift, the feedback network cutoff frequency must occur approximately one decade lower than the frequency of the slowest signal. Thus, for response to 0.1 Hz, the network should cut off at 0.01 Hz. This corresponds to a time constant of about 16 seconds.
Since the output of the transducer at low frequencies is likely to be very small, for example, on the order of a few microvolts, it is necessary for the feedback network voltage, that is, the integrating capacitor charge, to be very nearly at its steady-state value before useful information can be obtained. If the state of charge is altered by a large amount, such as 1 volt, the settling time will be very long, possibly extending to hundreds of seconds. Such a disturbance may occur due to the turn-on of circuit power, or to the application of a large, fast input signal to the amplifier. In either event, the resultant waiting time is at best inconvenient and in some applications, prohibitive. It is therefore desirable, if not mandatory, that circuit means be incorporated for rapidly establishing steady-state conditions, that is, to initialize the system. The present invention fills this need in a versatile and efficient manner, not only permitting the amplifier output to go immediately to its steady-state voltage but automatically compensating for input offset error. Thus, quiescent conditions may typically be established within a fraction of a millivolt in integrating amplifiers. Moreover, for amplifiers with time constants of several seconds, the initialization provided by the circuit configuration of the present invention takes place in a time period of the order of tens of milliseconds.